As a method replacing a conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor is described, for instance, in U.S. Pat. No.4,239,968, and is practically employed. In the method, a radiation image storage panel containing a stimulable phosphor (i.e., stimulable phosphor sheet) is used, and the method comprises the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (i.e., stimulating rays) to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. After the residual radiation image is erased from the radiation image storage panel, the panel is kept for the next radiographic process. Thus, the radiation image storage panel is generally employed repeatedly after the recorded image is erased.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to an object at a considerably small dose, as compared with the conventional radiography using a combination of a radiographic film and a radiographic intensifying screen. Further, the method is very advantageous from the viewpoints of conservation of resources and economic efficiency because the radiation image storage panel can be repeatedly used in the method, while the radiographic film in the conventional radiography is consumed for each radiographic process.
The stimulable phosphor absorbs and stores a portion of the radiation energy when exposed to a radiation such as X-rays, and shows stimulated emission when exposed to stimulating rays. In practical use, there are generally utilized phosphors giving stimulated emission within a wavelength region of 300 to 500 nm by exposure to stimulating rays within a wavelength region of 400 to 900 nm. As an example of the stimulable phosphor having been employed for a radiation image storage panel, there can be mentioned a rare earth activated alkaline earth metal fluorohalide phosphor.
The radiation image storage panel employed in the above-described method generally comprises a support and a stimulable phosphor layer provided on one surface of the support. However, if the phosphor layer is self supporting, the support can be omitted.
As the stimulable phosphor layer, there are known not only a phosphor layer comprising a binder and a stimulable phosphor dispersed therein but also a phosphor layer composed of only an agglomerate of a stimulable phosphor containing no binder which is formed by a deposition process or a firing process. Further, there is known a radiation image storage panel having other type of a phosphor layer which has a stimulable phosphor agglomerate having voids impregnated with a polymer material. In any of the above-described phosphor layers, the stimulable phosphor emits light (stimulated emission) when excited with stimulating rays, after having been exposed to radiation such as X-rays. Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the phosphor layer of the panel in an amount proportional to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored latent image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating the panel with stimulating rays. Thus stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
In the case that the phosphor layer is provided on a support, a film (i.e., protective film) is generally provided on its free surface (surface not facing the support) of the phosphor layer to protect the phosphor layer from chemical deterioration or physical shock.
The above-mentioned rare earth activated alkaline earth metal fluorohalide phosphor is advantageously employed in the recording and reproducing method, because it has high sensitivity and a radiation image storage panel using the phosphor reproduces a radiation image having high sharpness. Nevertheless, further improvements of the stimulable phosphor in its characteristics are desired.
The known process for the preparation of the rare earth activated alkaline earth metal fluorohalide phosphor comprises the steps of mixing its starting materials such as an alkaline earth metal fluoride, an alkaline earth metal halide (except metal halide), a rare earth fluoride and ammonium fluoride in a dry condition or in an aqueous suspension; and if desired, adding sintering inhibitor; firing the mixture and then pulverizing the fired product. Therefore, in the known preparation process, the pulverization step after firing is indispensable, and most of the thus prepared phosphor grains are made in a tabular shape. Consequently, the known rare earth activated alkaline earth metal fluorohalide phosphors are in the form of tabular grains.
In the known process for preparing a stimulable phosphor layer, the tabular stimulable phosphor grains and a binder polymer solution are mixed, and the obtained mixture is coated on a support, and dried. In thus prepared phosphor layer, the tabular phosphor grains are likely to be arranged to face parallel to the support surface as is shown in FIG. 1. By the use of thus prepared phosphor layer, a radiation image can be recorded and reproduced by means of the stimulating rays. However, the sharpness of the radiation image reproduced by the use of such phosphor layer is liable to lower, because most of the stimulating rays and most of the stimulated emission are likely to be scattered in the above phosphor layer in the level direction (i.e., direction parallel to the support surface; see the long arrow in FIG. 1).
In order to avoid such lowering of the sharpness of reproduced radiation image, it may be considered to use pseudo-cubic stimulable phosphor grains disclosed in Japanese Patent Provisional Publication No. 62(1987)-86086. However, the reproducibility of the disclosed preparation method of the pseudo-cubic phosphor grains is poor from a viewpoint of industrial production and therefore is inapplicable in industry.